1. Field of the Invention
This invention relates to the field of ion exchange, and more particularly to an electronic-hydroxide ion conductive membrane for pressure driven and electric power driven oxygen separation.
1. Background Information
A multitude of important chemical, environmental, medical, and electronics processing technologies require pure oxygen gas. For example, oxygen is used in semiconductor fabrication for chemical vapor deposition, reactive sputtering, and reactive ion etching. It finds wide application in health services, for resuscitation, or, in combination with other chemicals, for anaesthesia. Oxygen can be used for environmental benefit by reducing the sulfur emissions of oil refineries and helping pulp and paper manufacturers meet regulations relating to bleaching, delignification, and lime kiln enrichment. The high cost of pure oxygen limits the wide adoption of such beneficial processes in the chemical, electronics, and medical industries.
High-purity oxygen is now largely produced in cryogenic air separation plants, where the air is cooled down to the melting point of nitrogen (xe2x88x92210xc2x0 C.) and its components separated in large condensation columns. This process requires expensive, bulky equipment and high-energy consumption, which tends to militate against the use of oxygen to generate energy.
Over the last decade, a new technology has emerged for gas separation: selective membranes which pass only the desired components, such as described by H. J. M. Bouwmeester, A. J. Burggraaf, in xe2x80x9cThe CRC Handbook of Solid State Electrochemistry,xe2x80x9d Ed. P. J. Gellings, H. J. M. Bouwmeester, chapter 11, CRC Press, Boca Raton, 1997, which is incorporated by reference herein. The state-of-the-art membrane today is the Mixed Ionic-Electronic Conducting (MIEC) membrane, which relies on the transportation of oxide (O2xe2x88x92) ions to separate the oxygen from air. Although this approach may offer some advantages relative to cryogenic oxygen separation, practical application of the MIEC membrane is hindered by a number of drawbacks intrinsic to oxide (O2xe2x88x92) conductive membranes. These problems include: low oxygen throughput (typically caused by both low ionic conductivity and low surface oxygen exchange rate); relatively high operating temperature ( greater than 800xc2x0 C.); costly materials and costly fabrication; tendency to degrade over time; and system equipment that is relatively complex and expensive to build and maintain.
Thus, a need exists for an oxygen separation method and apparatus that addresses the problems associated with both cryogenic separation and oxide-based MIEC membranes.
An important aspect of the present invention was the realization that hydroxide ions (OHxe2x88x92), rather than oxide ions (O2xe2x88x92), may be utilized to shuttle oxygen molecules through a membrane at relatively high oxygen throughput. Although a hydroxide-conductive electrolyte has been used in the alkaline fuel cell since the first Apollo program in 1960, little attention has been paid to using a hydroxide electrolyte (e.g. KOH) as an oxygen separation medium. It was realized that by using optimal electrolytes, the hydroxide ion generally has higher conductivity than the oxide ion at any given temperature. It was also recognized that the surface oxygen exchange rate is higher in an alkaline electrolyte than in an oxide electrolyte, especially at low temperature.
The present invention provides, in a first aspect, an oxygen separating membrane that includes a backbone having a first surface and a second surface and an array of interconnected pores extending therebetween. A hydroxide ion conductor extends through the pores from the first surface to the second surface.
In a variation of this aspect, an electrical conductor extends through the pores from the first surface to the second surface. The electrical conductor is discrete from the ion conductor.
In a further variation of this aspect, at least one catalyst is disposed on each of the first surface and the second surface.
The present invention provides, in a second aspect, a method of fabricating a hydroxide conductive membrane. The method includes the steps of:
a) forming a porous backbone having first and second surfaces, by mixing a ceramic powder and fiber, molding the mixture under pressure, and sintering;
b) inserting an electrolyte into the pores to provide a hydroxide conductive pathway extending between the first and second surfaces; and
c) wherein the first and second surfaces respectively comprise an anode and cathode, and the membrane is ionically conductive therebetween to reduce oxygen at the anode and reoxidize hydroxide ions at the cathode.
A variation of this aspect includes the step of
d) metallizing the pores to provide an electrically conductive pathway extending between the first and second surfaces, wherein the membrane is ionically and electrically conductive between the anode and the cathode.
The above and other features and advantages of this invention will be more readily apparent from a reading of the following detailed description of various aspects of the invention taken in conjunction with the accompanying drawings.